Who was he? I’m talking about Augustine of Hippo (the more famous one) not of Canterbury. Augustine of Hippo was a monk and later Bishop of Hippo (he was originally from Carthage). His early life was spent either in sinful living or involved with various cult or cult-like philosophical movements. Eventually he converted to Christianity and wrote a dazzling amount of material, revitalizing the faith in his time and setting the stage for the Institutional Church to survive once the Roman Empire began to crumble even further than it had. The biggest controversy of his day was against the Pelagians, who taught that it was possible to live a perfect life and so, apart from Grace, gain access into heaven. Augustine opposed them regularly. Slightly less influential were the Manichees and other Gnostic groups, Augustine having formerly been part of the Manichees. He systematically disproved their positions.
Where might I have heard of him? Aside from a monastical order (the Augustinians), many people are required to read his book Confessions or The Confessions, even in secular literature courses. The book is part biographical, where he “confesses” his sever sins of youth, and part philosophical reflection, where he “confesses” the greatness of God.
Fun Fact: There is still a considerable amount of debate about how to pronounce his name in English (though the shorter vowel sounds seem to be winning). So it is either pronounced like the eighth month of the year with ‘teen’ on the end (which shows a clear anglicanization/germanization of the pronounciation) or pronounced like the adjective ‘august’ with ‘tin’ on the end (which may approximate the Latin more closely). Also, he stated he was never very good at Greek or Hebrew, and chose to rely almost entirely upon Jerome’s Latin translation of the Bible.
Why was he important? Aside from the things I’ve already mentioned, Augustine pioneered the idea of a systematic theology, of thinking of the Kingdom of God as being the Church, and of providing a more philosophical voice to theological discussion. His ideas strongly influenced many in the Reformation, not least of all Martin Luther who began as an Augustinian monk, and continue to have strong influence in all Christian denominations today. His other works, are still widely read. Especially The Enchiridion, and The City of God.